The present invention concerns a process for the production and improved separation of aromatic C8 hydrocarbons from a feed which is rich in para-xylene, such as a C8 cut from the Mobil toluene disproportionation process.
It has long been recognized that certain zeolitic materials catalyse certain hydrocarbon conversions as described, for example, in U.S. Pat. No. 4,305,808. Particular hydrocarbon conversions which have been claimed are alkylation, transalkylation and disproportionation. Toluene disproportionation is claimed, for example, in U.S. Pat. Nos. 4,052,476; 4,007,231; 4,011,276; 4,016,219 and 4,029,716.
In their article in Oil and Gas Journal vol. 69, No 48 (1971), Grandio et al. describe a process for the disproportionation of toluene in the liquid phase using zeolitic catalysts in the absence of hydrogen.
It should be noted that in the first known processes, the composition of the xylenes obtained corresponds to the thermodynamic equilibrium at the operating temperature, namely 20% to 25% of p-xylene of the total of xylenes and ethylbenzene.
More recent patents, for example U.S. Pat. No. 4,380,685, describe alkylation, transalkylation and paraselective disproportionation of substituted aromatic compounds to form dialkylbenzenes. In the case of toluene disproportionation, the p-xylene concentration in the effluent produced exceeds the thermodynamic equilibrium. The catalyst used is based on zeolites characterized by a constraint index of 1 to 12, a silica/alumina ratio of at least 12/1 and comprising various metals and phosphorous; examples of the zeolites used are ZSM5, ZSM11, ZSM12 and ZSM35. Further, those catalysts must be treated to restrict diffusion of ortho- and meta-xylene through the pores of the crystal and to reduce re-isomerization of the p-xylene formed. This treatment essentially consists of precoking under well defined conditions which may be accompanied by addition of a small quantity of an oxide which is difficult to reduce (antimony, phosphorous, boron or magnesium), or by a surface treatment. Examples of methods for the preparation of such an effective catalyst are described in U.S. Pat. No. 5,173,461.
The process using those catalysts, termed the MSTDP process, has been described in two publications:
Selective toluene disproportionation process proven at Italian refinery, Gorra F., Breckenridge L. L., Guy W. M., Sailor R. A., Oil and Gas Journal Vol. 90, No. 41, 60-67 (1992) and in "Mobil's toluene to PX process proves itself", Mobil Research and Development Corp., European Chemical News Vol. 54, No. 1418 (1990).
These publications describe the production of xylenes containing 80-95% of p-xylene for a conversion of 30% of toluene per pass.
A number of patents claim this process and describe catalyst preparations. Examples are European patent EP 26,962; U.S. Pat. Nos. 4,260,843; 4,274,982; 4,908,342; 5,173,461; and WO 93/17987.
In all cases, the disproportionation effluent has a simplified composition of close to: 70% toluene--15% benzene--15% xylenes. The composition of the xylene fraction is 85% p-xylene, 15% o-+m-xylene and ethylbenzene.
The present invention relates to an improved process for the separation of a C8 aromatic mixture to obtain p-xylene of sufficient purity for the production, for example, of terephthalic acid. One known method for the separation of p-xylene from a C8 aromatic cut consists of carrying out fractional crystallization; examples of existing processes are those or Chevron, Krupp, Amoco, Maruzen and Arco U.S. Pat. Nos. 3,177,255 and 3,467,724). Those processes use the following procedure: the feed containing at least 20% of p-xylene is cooled to between 50.degree. C. and 70.degree. C. to cause crystallization, the crystals containing 85% to 90% of p-xylene and the mother liquors, 7% to 8%; the crystals are melted again and recrystallised at -10.degree. C. After filtering the crystals and washing with toluene, for example, 99.5% pure p-xylene is obtained.
A further method for the separation of p-xylene from an aromatic C8 mixture is the liquid chromatographic method termed a simulated counter-current method claimed in U.S. Pat. No. 2,985,589 which uses zeolites to selectively adsorb the p-xylene. The Parex and Aromax processes use this method or a simulated co-current method (U.S. Pat. No. 4,402,832).
The advantages and disadvantages of those processes have been widely studied and described for different cases.
The prior art is also illustrated in U.S. Pat. No. 5,329,060 and EP-A-0 531 191 which describe a combination of adsorption, crystallization and isomerization steps for aromatic C8 cuts to produce very high purity para-xylene.
The peculiarity of the mixture obtained at the outlet from the paraselective disproportionation reactor in the MSTDP, after separation of the lighter hydrocarbons by distillation, is that it is particularly rich in p-xylene as it contains 75% to 85% of p-xylene.
This mixture can be treated in different ways:
1. High temperature crystallization
This means a temperature which is, for example, in the range +10.degree. C. to -25.degree. C. The yield of pure p-xylene is close to 83% when the concentration of p-xylene in the C8 cut in the effluent is 80 weight %. This yield is limited by the fact that the mother liquor still contains more than 40% of para-xylene due to the liquid-solid thermodynamic equilibrium.
2. Two-temperature crystallization
In this case, a first crystallization at a temperature in the range +10.degree. C. to -25.degree. C. leads to the production of pure para-xylene, the mother liquor is recrystallized at a lower temperature (-50.degree. C. to -70.degree. C.), the p-xylene-rich crystals are recycled to the feed to the first crystallization, the mother liquor from the second crystallization contains no more than about 10 weight % of p-xylene.
The global yield of para-xylene from the two-stage crystallization method for an initial feed of 80 weight % of para-xylene is about 97 weight %.
The disadvantage of this process is high energy consumption.